The major objective of the proposed research is to gain a better understanding of the hormonal regulation of the rat corpus luteum during pregnancy and pseudopregnancy. Particular attention will be given to the local action of estrogen within the corpus luteum and to the role of the thecal lutein cell in the normal activity of the corpus luteum. The importance of the thecal lutein cell will be evaluated by investigating the functional capabilities (progesterone and 20Alpha-dihydroprogesterone synthesis, estradiol synthesis, aromatase activity, prolactin and LH receptor) of luteal tissue formed in the absenceof thecal lutein cells. This will be accomplished by the method of granulosa cell homotransplantation, recently developed in this laboratory. The concept that estrogen, formed in the corpus luteum, mediates certain actions of LH will be examined further by: a) determining if LH can stimulate estrogen synthesis in the corpus luteum; b) role of estrogen in mediating LH action to maintain progesterone synthesis and in predisposing luteal regression; c) role of estrogen in maintaining optimal luteal blood flow and in maintaining the normal morphology and ultrastructure of the luteal vascular system. For some of these studies, hypophysectomized rates with homotransplanted pituitary will be used. To determine direct effects of hormones on the corpus luteum, we will use rats with ectopic orpora lutea located beneath the kidney capsule. Blood flow in the corpus luteum will be determined by the reference sample radioactive microsphere method. Finally, certain aspects of luteal regression will be investigated by determining, a) if regressing luteal cells produce substances inimical to progesterone synthesis using transplants of mixed populations of regressing and non-regressing luteal cells; and, b) if the luteolytic effect of LH is via loss of luteal estrogen synthesis.